This invention relates to a method of manufacturing calcium carbonate single crystal (calcite) widely used for an optical polarizer, etc.
Single crystals of calcium carbonate (CaCO.sub.3) are suitable for optical uses and, for this purpose, natural calcite crystals are currently used. Calcite single crystals exhibit double refraction of incident light. Calcite has a high refractive index and is used as a polarizing prism in an optical apparatus. Because of recent advances in the design of apparatus such as laser optics and an optical communication apparatus, there is an increased demand for a material with excellent optical characteristics. In this regard, calcite single crystal is an ideal material, and is expected to be more and more in demand.
Calcite single crystal has been obtained only from natural sources because it is not yet being industrially synthesized. For commercial use, natural calcite must be colourless and transparent, must have no bubbles or cracks, no twining, and no internal strains, and must be greater than a certain size. However, calcite single crystal that will meet this requirement is found only in limited quantities in the Republic of South Africa, Mexico, etc., and reserves are running low.
There have been experiments to synthesize calcium carbonate single crystal. One method is crystallization from a solvent, another is the synthesizing from a gel, a third is crystallization from a flux or melt, a fourth is hydrothermal synthesis, and recently an FZ method under high pressure have been suggested. However, optical characteristics such as transparency of resultant crystals are not entirely satisfactory due to defects such as impurities, mixing, dislocations, inclusions, or internal strains.
Among the methods tried for the manufacture of calcium carbonate single crystal, hydrothermal synthesis is most similar to the process by which natural calcite is grown as a hydrothermal ore deposit. Therefore, hydrothermal synthesis can produce a desired calcium carbonate single crystal with characteristics similar to natural calcite.
The hydrothermal synthesis process for manufacturing calcium carbonate single crystal utilizes an aqueous solvent at held a fixed temperature and pressure in an autoclave. Alkaline aqueous solutions such as sodium hydroxide (NaOH) or alkali carbonate aqueous solutions such as sodium carbonate (Na.sub.2 CO.sub.3), potassium carbonate (K.sub.2 CO.sub.3), etc. are generally used as the aqueous solvent. This method for growth of calcium carbonate single crystal is essentially a modification of conventional growth technology for artificial crystals. Under the following conditions:
Solvent--6 mol K.sub.2 CO.sub.3 aqueous solution PA1 Temperature--410.degree. to 445.degree. C. PA1 Pressure--1720 atmospheres PA1 Growth speed--50 .mu.m/day
about 3 mm growth layer of a calcium carbonate single crystal has been obtained.
The above described hydrothermal synthesis is disclosed in D. R. Kinlock H, R. F Belt, R. C. Puttbac H, Journal of Crystal Growth 24/25 (1974) 610-613.
The inventors have invented a new method and filed U.S. patent application Ser. No. 840,670) with respect to a method of manufacturing calcium carbonate single crystal grown by hydrothermal synthesis using a chloride aqueous solution.